Heat generating components in small and thin electronic devices for mobile use (mobile devices), such as smart phones and tablets, for example, are cooled using thermally-conducting sheets, including metallic plates and thermal diffusion sheets, for example. Examples of such metallic plates are thin plates of copper, aluminum, magnesium alloy, and a laminate thereof, for example. The thermal conducting performance thereof depends on the thermal conductivity of the material. Examples of thermal diffusion sheets are graphite sheets, which have a thermal conductivity of about 500 to 1500 W/mK. Heat transfer is not performed sufficiently with such a level of thermal conductivity when the heat generating components generates a large amount of heat.
For efficient transfer and diffusion of a larger amount of heat, use of heat pipes could be effective. Heat pipes are heat transfer devices which use latent heat of vaporization of a working fluid, instead of thermal conduction of the material. For example, a heat pipe with a diameter of about 3 to 4 mm is equivalent to a thermally-conducting sheet with a thermal conductivity of about 1500 to 2500 W/mK, that is greater than that of thermal conducting sheets. For more efficient heat transfer, pipes as the heat transfer pipe increase in diameter. This causes a problem in mounting such a heat pipe on a device, and heat pipes are not increasingly applied to mobile devices.
In such a case, heat pipes may be flattened. However, flattening heat pipes will block working fluid from flowing smoothly in the pipe, thus reducing the heat transfer ability. On the other hand, in a loop heat pipe, the channel of working fluid of vapor phase and the channel of the working fluid of liquid phase are independent of each other, and the working fluid flows in one direction. The resistance to flow of the working fluid is small in the loop heat pipe, enabling efficient heat transfer compared with heat pipes in which the working fluid of liquid phase and the working fluid of vapor phase reciprocate within the pipe.
Such a loop heat pipe is considered to be suitable in mobile devices.
Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication Nos. 2009-236362 and 2004-190976.